Ligands, quadridentate coordination complexes

All of the complexes structurally characterized have either a square planar or a square pyramidal structure with a planar arrangement of the four donor atoms. Complexes having general formulas [NiL]Y2 and [NiXL]Y (L = quadridentate ligand X = coordinated anion Y = uncoordinated anion) are invariably low-spin as one can easily predict on the basis of the high nucleophilicity of the donor atoms.1389... 

The formation and dissociation kinetics for the complexation of Cu, Zn, Co and Ni with the quadridentate l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclotetradecane to give five-coordinate species have been reported.1203 The rate order (Cu > Zn > Co > Ni) is the same as that for H2O exchange, but the rates are much slower, probably owing to conformational changes occurring in the ligand. The dissociation is acid-catalyzed the five-coordinate species are found to be much less kinetically inert than four- or six-coordinate complexes. No macrocyclic effect was observed. 

Metal Containing Tetradentate Formazans. Formazans with two complex-forming groups in the 2- and 2 -positions of the N1- and N5-aryl substituents are potential quadridentate ligands. They form 1 1 coordination complexes with four- and six-coordinate metals ... 

In general, the overall formation constants of chelates are higher than those of similar simple coordination complexes. Even if the constants were the same, however, there is a tremendous difference in the extent of comple-xation that can be best explained by an example. Divalent copper ion, having a coordination number 4, can react with four monodentate ligands, L, or one quadridentate chelating agent, L. Let us assume that the overall formation constant in each case is lO and that the equilibrium concentration of ligand in each case is 10 M. Then,... 

Cu (L)Cl][NTf2] (L = bis(2-dimethylaminoethyl)-((l-methylimidazol-2-yl)methyl)-amine), mpt. 108°C, extending the ligand palette for transition metal coordination complex ionic liquids to include quadridentate tripodal chelates. 

Quadridentate ligands produce 3, and in some cases 4, rings on coordination, and so even greater restrictions on the stereochemistry of the complex may be imposed by an... 

The complexes ML2-nH20 (M = Zn or Cd H2L = diformyl- or diacetyl-hydrazine) have also been synthesized, and are dimeric or polymeric with the quadridentate hydrazine acting as a bridging ligand.158 159 A normal coordinate analysis of diformylhydrazine and its zinc complex with deprotonated ligand has been reported.160... 

Cyclization of coordinated primary amines on to coordinated aminoacetone has also been investigated in bis(l,2-diaminpethane)cobalt(ni) complexes, and the results show selectivity with respect to attack of the monoamine or 1,2-diaminoethane (equation 38).218 A similar complex with two coordinated aminoacetone molecules undergoes the same type of stereoselective kinetic template reactions and yields complexes primarily of a new quadridentate ligand (Scheme 48).219... 

Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

The types of model systems in which 5-coordinate species have been invoked range from the relatively electron-deficient cobaloximes to the relatively electron-rich Schiff base complexes, where the corrin ring in cobal-amins is replaced by two dioximato ligands or by a quadridentate N202 dinegative Schiff base ligand, respectively. These two classes of models, in fact, appear to bracket many characteristics of cobalamins since the cobaloximes are too electron-deficient and the Schiff base complexes are too electron-rich [136], As such, an understanding of the differences in properties between these two types... 

A class of synthetic metalloporphyrins which are conveniently studied in aqueous solution derives from coordination compounds of the quadridentate ligand a,(S,7,5-tetra(4-pyridyl)-porphine. Although a method of synthesis for the zinc complex has been published,1 the yield is often poor and the method unreliable. If the same reaction is allowed to proceed under... 

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